Recently, global warming caused by burning fossil fuel and increase in energy demand associated with population growth have been serious problems influencing existence of human being. Needless to say, sunlight has fostered terrestrial environment and supplied energy to all the living things including human being since ancient times. Thus, recently it has been considered to utilize sunlight as an energy source that is infinitely available and clean without emission of any harmful substances. In particular, attention has been paid to photoelectric devices converting light energy to electric energy, so-called solar cells, as a powerful technology.
As materials for generating electromotive force in solar cells, there have been used silicon in a single crystalline, polycrystalline, or amorphous form and inorganic semiconductors composed of compounds such as CuInSe, GaAs and CdS. Solar cells using these inorganic semiconductors attain relatively high energy conversion efficiencies, 10% to 20%, and therefore they are widely utilized as remote power supplies and auxiliary power supplies in portable small-sized electronic apparatuses. In light of the purpose mentioned in the beginning, namely, for preventing damage to the global environment by reducing consumption of fossil fuel, however, solar cells using inorganic semiconductors cannot be regarded to be sufficiently effective at the present stage. This is because such solar cells using inorganic semiconductors are produced by a plasma-assisted CVD method or a high-temperature crystal growth process, which means that a large quantity of energy is required to produce the devices. Further, such devices contain Cd, As, Se or the like, which possibly have harmful effects on the environment, causing possibility of environmental damages on disposal of the devices.
There have been proposed organic solar cells in which organic semiconductors are used as photovoltaic materials capable of improving the above issue. Organic semiconductors have excellent characteristics: a wide variety of materials are available; toxicity is low; production cost cutting is possible due to high workability and productivity; and they can be readily flexibilized due to the flexing nature; and others. For this reason, organic solar cells have been actively studied toward commercialization.
Organic solar cells are largely classified into semiconductor type and dye-sensitized type. The semiconductor type is classified into two classes, Schottky barrier type and pn junction type, according to the mechanism in dissociation of photo-generated charged pairs. Schottky barrier solar cells utilize internal electric field due to Schottky barrier induced in a junction plane between an organic semiconductor and a metal (see Non-patent Document 1). While such Schottky barrier solar cells can attain relatively high open-circuit voltage (Voc), they have a drawback that the photoelectric conversion efficiencies tend to decrease with an increase in irradiation light intensity. Further, production of Schottky barrier solar cells is generally rather difficult since thin films are required to be formed by various kinds of vapor deposition methods.
The pn junction solar cell, which utilizes internal electric field generated in a junction plane between a p-type semiconductor and an n-type semiconductor, include organic/organic pn junction type, in which organic materials are used for the both semiconductors, organic/inorganic pn junction type, in which an inorganic material is used for either of the semiconductors, and others. The conversion efficiencies of such pn junction solar cells are relatively high but not sufficient (see Non-patent Document 2). In many cases, it is also required to form films by vapor deposition methods as in the case of Schottky barrier solar cells, which impedes improvement of productivity.
On the other hand, in 1991, Gratzel et al. in Switzerland reported a dye-sensitized solar cell using a thin film of porous titanium dioxide with large surface area containing ruthenium bipyridinecarboxylic acid dye adsorbed on a surface thereof as an electrode, and this report attracted great attention (see Non-patent Document 3). However, a problem was pointed out for the use of an electrolytic solution and iodine, and commercialization has been hard to progress. Solidification of the electrolytic solution component could evidently make a significant advance toward commercialization. Although various methods have been tried for the solidification, the photoelectric conversion efficiencies of such systems are still lower than those of wet-type solar cells using electrolytic solutions. For example, conductive polymers such as polyaniline, polypyrrole, polythiophene, and the like, have been examined, but the conversion efficiency is low in any case (see Non-patent Document 4). For such a solidification method of replacing the electrolytic solution component by a p-type semiconductor layer working as a hole-transporting layer, there may be pointed out structural similarity to a pn junction solar cell except that the n-type semiconductor electrode is a porous material and that an organic dye layer is provided.
Furthermore, it is known that organic semiconducting compounds considered to be valuable for solar cells exhibit different properties, for example, photoelectric characteristics (electromotive characteristics), depending on crystal forms, and that only specific crystal forms exhibit excellent photoelectric characteristics. To selectively generate a crystal form in a film forming step by a vapor deposition method, improvement has been attempted by controlling the substrate temperature, but it is difficult to obtain a semiconductor layer in the desired crystal form having good photoelectric characteristics.    Non-patent Document 1: R. O. Loutfy et al., J. Chem. Phys. (1979), Vol. 71, p. 1211    Non-patent Document 2: C. W. Tang, Appl. Phys. Lett. 48 (2), 13 Jan. 1986, p. 183    Non-patent Document 3: B. Oregan, M. Gratzel, Nature, vol. 353, p. 737 (1991)    Non-patent Document 4: K. Murakoshi, R. Kogure, Y. Wada, and S. Yanagida, Chemistry Letters, 1997, p. 471